It is known to produce plastic (or resin-based) bottles by injection blow molding. Injection blow molding comprises the steps of first producing an intermediate resin article having a shape similar to that of the final resin article. This intermediate resin article is often referred to as the parison, or preform. The parison is usually formed by an injection molding process whereby molten resin is forced into a cavity defined by the space between a moveable core rod and the parison mold into which the core rod is placed. Once the injection molding part is complete, the thus-formed parison is transferred from the parison mold to the blow mold via the moveable core rod on which it rests.
The parison is next transferred, via the core rod, to the blow mold. The longitudinal axis of the core rod is generally parallel to the plane of separation of the blow mold halves. The blow mold cavity has the shape of the outside of the final bottle product. The parison is then inflated, often through the introduction of air through the core rod, and caused to expand to the shape of the blow mold. Typically, because the parison wall is effectively stretched during the blowing process, the final product wall thickness is less than the wall thickness of the parison.
The blow mold is typically comprised of two halves, each of which moves in a substantially opposite direction with respect to the other. The contact area where the two mold halves meet and later part (after the blowing is complete) is usually called the parting line. The parting line is usually substantially parallel to the longitudinal axis of the core rod. Where special product shapes are desired, however, it is sometimes necessary to have a parting line that is not linear or completely parallel to the core rod axis.
Where special bottle shapes are desired, and thus the parting line is not linear, it becomes important to insure that the core rod can be both transferred into the location between the open blow mold halves and removed from the same position with the blown bottle in place on the core rod. Because of conventional machinery, it is equally important is that the blown bottle be able to be stripped from the core rod in a direction parallel to the longitudinal axis of the core rod.
Because the blown bottle must be stripped from the core rod in a direction parallel to the axis of the core rod, it can be difficult to blow mold bottles having an undercut shape. This is especially true where it is important that the product bottle have a consistent wall thickness throughout. Where consistent wall thickness is desired, it is preferred that the parison be positioned in (or as close as possible to) the geometric center of the blow mold cavity. Where the parison is not in the middle of the blow mold cavity, the distance that the molten plastic comprising the parison must travel when blown is greater in some parts of the mold than in others. This incongruity in the distance stretched results in a bottle having varying wall thickness in its different regions.